All the light we cannot see

How do we colourise photos of space?

Hubble and Webb have been blessing our feeds with beautiful and stellar images of the Universe. Anyone who looks at them cannot deny the awe and wonder it fills you with. But, do they actually look like this? More precisely, if we were to observe this Universe with the naked eye would we actually see the bodies as their images show them to be? Are they as colourful as is shown or are we somehow manipulating them to see what we understand?

All the colourful pictures that we see on our devices use three colours as their primary colours, i.e.; red, green and blue. These colours in various proportions make other colours. For example, in a hexadecimal representation of colours, we have six digits. The first two digits represent a red value, the next two are the green value, and the last two are the blue value.

Some common hex codes and their corresponding colour

The human eye consists of six million photoreceptor cells in the retina, called the cones. There are three types of cones or receptors, the short-S (blue), medium-M (green), and long-L wavelength (red) sensitive cones. These cones are present in different quantities. The short wavelength cones make for about 10% of the cones. It responds most to blue-wavelength light peaking at 420 nm. The medium wavelength cones respond most to light of yellow to green, and peak at about 530 nm. The long wavelength or red-sensing cones make for about 60% of the cones. It responds most to the longer wavelengths peaking at about 560 nm. An important thing to be noted is the peaks aren’t the same for all the individuals. It might lie in the range of 420-440 nm, 534-545 nm, and, 564-580 nm respectively for different individuals.

Responsivity vs Wavelength (nm)

This is the guiding principle used in colouring black-and-white images. We use a process called broadband filtering which is essentially taking pictures of objects with different filters and then recombining them to get the desired image. For example in the picture given below, we have taken a black-and-white photograph of the flowers. The same black-and-white photography is done using red, green, and blue filters. The relative absence and presence of a particular colour in the given photographs predict the actual colour present. This calculation is done on a pixel-by-pixel basis and we obtain a colourful image.

Broadband filtering at play

The images taken by Hubble and Webb are in black and white. The main purpose of these telescopes is to measure the brightness of light reflecting off  of objects in space which is clearest in black and white. These images are then digitally coloured. We take images filtering various wavelength ranges and then recombine the image to get the picture.

Scientists also use colours to map out how different gases interact to form galaxies and nebulae. A process called narrowband filtering is used to capture specific wavelengths of light. Hubble can record very narrow bands of light coming from individual elements like oxygen, carbon, and hydrogen. We can then use colours to track their position in an image.

Narrowband filtering of Hydrogen, oxygen, and Sulphur

The most common application of narrowband filtering is studying the formation of stars and galaxies. The filters isolate light from hydrogen, sulfur, and oxygen, the three key building blocks of stars. This is not a true colour image. It is more of a colourised map. The characteristic wavelengths of hydrogen, sulphur, and oxygen are 656.2 nm, 672 nm, and 495.9 nm respectively. Hydrogen and sulphur are naturally seen in red light, and oxygen is seen in blue. These correspond to the colours red, red, and cyan. To get a better image the wavelengths are adjusted and assigned their places as red, green, and blue according to their chromatic order. Sulphur is denoted by red colour, hydrogen takes green and oxygen is shown by blue.

Pillars of Creation in True and false colours

What about infrared light? We do know the primary wavelengths Webb is working in are in the infrared region. How do we colourise invisible light?

In infrared light too, a similar process is followed. We assign different colours to different elements. Take their images through broadband and narrow band filtering and then recombine them to show the stunning images we are presented with.

Infrared photography of Helix Nebulae

Have we been duped? Are colours even real? Are we artificially colouring the Universe to make it look more beautiful than it actually is?

Well, yes and no. It’s true that if we are presented with actual images of the galaxies taken, they may appear bland and boring. But, the biggest fact we aren’t paying heed to is how limited we as humans are in our perception of light. We see a small part of the electromagnetic spectrum and call it visible light. If we could see the complete spectrum imagine how colourful we would find the Universe to be! We could see temperatures, the all-surrounding microwave background radiation, and telephones communicating in various radio waves. The very thought about it makes me trippy. What about you? So, we have been duped ever so slightly to understand the Universe in a colour language we know. In reality, the Universe is much more colourful than we can ever comprehend.

I hope you liked this article. Drop a comment and tell me what you thought about it. See you soon.

Auf Wiedersehen!

Looking into the Past (Part 2)

  

James Webb Space Telescope

Welcome back, readers. If this is the first article you have opened, I’d highly recommend reading the first part of this article. While the former was about the engineering elements of JWST, this article is mostly focused on the physics surrounding JWST.

While discussing the parts of JWST, we talked about the Sun Shield. For the sun shield to successfully protect us from the heat of the Earth, Moon and Sun it should be placed 1.5 million kilometres from Earth. The telescope is placed at a point in space called Lagrange Point 2. It is one of the five Lagrange points.

The five Lagrange Points

Lagrange points are locations in space where both the Earth and the Sun exert a gravitational pull in the same direction. An object at this point has two gravitational forces pulling on it to make it move in a circle. This not only allows it to orbit the sun with a higher velocity, but it also keeps it at a fixed point relative to our planet. The JWST orbits the sun instead of the Earth. We want the JWST to be both further from the sun and complete a solar orbit in the same amount of time as the Earth? To make it easier to control, the telescope would also have to remain in the same position relative to the Earth. Here, the Lagrange point comes into the picture.

Why Infrared?

A question that keeps appearing, again and again, is why we have chosen infrared as our desirable wavelength for detection?

Infrared waves have a longer wavelength than visible light. It means it can easily penetrate dust clouds and we can observe more far-off objects. Moreover, the light of galaxies that are billions of light years away from us travels to us through an ever-expanding space. This stretches the wavelength of visible light into the infrared region. Near-infrared light reveals the formation of galaxies and due to its longer wavelength, it can pass through the dust layers that enclose the newborn stars. Mid-infrared light peers through the cold, dusty regions where stars form, and reveals how massive stars and black holes shape their surroundings.

Southern Ring Nebulae in Near and Mid-Infrared Regions

Moreover, various types of celestial objects – including the planets of the solar system, stars, nebulae, and galaxies give off energy at wavelengths in the infrared region of the electromagnetic spectrum.

A follow-up question that comes to mind is if we are so adamant about looking at the Universe’s past. Why not take it to extremes and study light in microwave regions? As far as we know the Cosmic Background Radiation is in the Microwave region. Won’t it enable us to look further into the past, right to the origins of the Universe?

Well, the answer is yes and no. This means it is a little complicated…

Infrared and microwave imaging both have their own advantages and are useful for different purposes in astronomy. Infrared imaging is preferred over microwave imaging in many cases because it provides higher spatial resolution and can reveal more details about the characteristics of astronomical bodies. Infrared radiation has shorter wavelengths than microwaves, which means it can be used to study smaller features in the Universe. In addition, infrared radiation is absorbed and emitted by many astronomical objects, which makes it useful for studying the temperatures and compositions of these objects. Spatial resolution is inversely proportional to the observing wavelength. The higher the wavelength, the lesser would be its resolution and vice-versa.

Different Spatial Resolutions

Microwave imaging is preferred in some cases because it can penetrate through clouds of gas and dust that may obscure infrared radiation. Microwaves are also less affected by atmospheric turbulence, which can distort images taken at other wavelengths. This makes them useful for studying objects behind dense clouds of gas and dust, such as the centres of the galaxies. Microwave imaging is also useful for studying the cosmic microwave background (CMB), which is the leftover radiation from the Big Bang.

Cosmic Microwave Background

In summary, both infrared and microwave imaging have their own advantages and are used for different purposes in astronomy. Infrared imaging is preferred due to its higher spatial resolution and ability to reveal details about astronomical objects, while microwave imaging is useful for studying objects behind dense clouds of gas and dust and for studying cosmic microwave background radiation.

The Sun in different wavelengths

The next order of business is asking what makes Webb different from Hubble. In what ways is Webb an upgrade or a downgrade in comparison to Hubble?

Both JWST and Hubble are reflecting telescopes that conceptually work the same. Light reflects off a large primary mirror onto a secondary mirror, which sends it back through a hole in the primary mirror and into science instruments for analysis.

The basic difference between the two telescopes is the wavelengths they work at. Hubble, which is in Earth's orbit, is optimized for visible and ultraviolet wavelengths of light. 

Karina Nebulae as seen from Hubble and Webb

Webb essentially orbits the sun and is situated at Lagrange point 2 which is 1.5 Million Kilometres away from Earth as opposed to Hubble which is 535 Kilometers. This makes maintenance work on Hubble easier than it is for Webb.

Distance of Hubble and Webb from Earth

Hubble’s single mirror is 2.4 meters (7.9 feet) wide, whereas JWST’s segmented honeycomb-shaped mirror is 6.6 meters (21.7 feet) across. JWST has the largest mirror ever flown in space. Tiny actuators shape each mirror to provide a single, sharp image for the telescope’s science instruments to digest. Even though Webb’s mirror is a lot bigger in size and a hundred times more powerful, it is still 113 Kilograms lighter than Hubble’s mirror.

Hubble vs JWST Primary Mirror

The two telescopes also have very different cooling requirements. Hubble does not have as sophisticated cooling needs as JWST.

How has Webb changed and challenged our understanding of Physics?

A picture that made big news two months back was captured by Webb. It showed multiple galaxies that were formed way before our current understanding of Physics would permit. These galaxies grew way too large way too soon after the big bang. News spread everywhere. Webb has broken the Big Bang theory. So, do these pictures put our current understanding of the Big Bang and the Standard Model into question?

The oldest galaxies captured by James Webb

As fascinating as it might be, the answer is no. Recently, researchers took a closer look at the data and concluded that the distant galaxies discovered by Webb are in fact in perfect compatibility with our modern understanding of Cosmology.

As things go this might not even be the final answer and astronomers may find galaxies at very large distances with very large masses that puts our understanding of Physics into question.

But, we must always remember that in science, it’s always important to keep an open mind. For now, we can keep the exaggerated claims to rest.

I hope you liked this article and enjoyed reading it as much as I enjoyed writing it. Goodbye for now.

Auf Wiedersehen!

Looking into the Past

James Webb Space Telescope (JWST)

"…the laws of physics, carefully constructed after thousands of years of experimentation, are nothing but the laws of harmony one can write down for strings and membranes. The laws of chemistry are the melodies that one can play on these strings. the universe is a symphony of strings. And the “Mind of God,” which Einstein wrote eloquently about, is cosmic music resonating throughout hyperspace." 

- Michio Kaku

The biggest news of 2021 which gravitated not only the experts but has been the talk of the town ever since its launch is none other than the James Webb Space Telescope. The 8.8 billion dollars telescope with an estimated operating cost of 1 billion dollars was launched on December 25, 2021. The telescope deemed to be the successor of Hubble and its counterpart, has delivered images that have left the world in awe and put our understanding of Physics into question. So, let’s dive into it.

James Webb Space Telescope

Webb is a spectacular example of engineering and physics. We have long discussed how light gets red-shifted as it travels longer and longer distances in the Universe. Visible light emitted from these far-away bodies gets red-shifted into the infrared region by the time it reaches Earth, becoming invisible. Lucky for us JWST has been designed to work primarily in the infrared region.

Red-shift due to the expansion of the Universe

The telescope is broadly divided into its parts, namely, Optical Telescope Element (OTE), Integrated Science Instrument Module (ISIM), a sun shield and a Spacecraft Bus. We would look into each of these one by one.

Parts of JWST

A. The Optical Telescope Element (OTE) consists of the mirrors and the backplane. It is the eye of the observatory. It gathers the light coming from space and provides it to instruments placed in ISIM. The OTE consists of JWST’s segmented honeycomb-shaped mirror. It is the largest mirror ever flown in space. It consists of 18 hexagonal segments with each segment about 1.32 meters across. Each segment is made out of lightweight beryllium and coated with a thin layer of gold, making it more sensitive to infrared light. The hexagonal shape of the mirror helps in folding the mirror on Earth and then unfolding it in space. While in space, the focus of the mirror is adjusted on the secondary mirror with an accuracy of 1/10000th the thickness of a human hair! The order of various mirrors is the primary mirror, secondary mirror, fine steering mirror and infrared detector.

Optical Telescope Element

B. The light is collected on the secondary mirror. The detector converts these photons into their supposed electric voltages which are then processed to yield the spectacular pictures we have been getting. The second mirror consists of the Integrated Science Instrument Module (ISIM), which further contains instruments such as a Near-Infrared Camera (NIRCAM), Near-Infrared Spectrograph (NIRSPEC), Fine Guidance Sensor/ Near Infrared Imager and Slit-less Spectrograph (FGS/NIRISS), and Mid-Infrared Instrument (MIRI). We would discuss them briefly here: 

Integrated Science Instrument Module (ISIM)

1. The Near Infrared Camera (NIRCAM) is Webb's primary imager that covers the infrared wavelength range of 0.6 to 5 microns. Equipped with ten sensitive detectors it detects short wavelength channels (0.6 - 2.3 microns) and long wavelength channels (2.4 - 5 microns).

NIRCAM detects light from the earliest stars and galaxies in the process of formation, the population of stars in nearby galaxies, as well as young stars in the Milky Way, and Kuiper Belt objects.  NIRCAM is equipped with coronagraphs. They filter bright light and help in detecting fainter sources of light like the ones coming from exoplanets. With the coronagraphs, astronomers hope to detect planets orbiting nearby stars.

A basic Coronagraph

While NIRCAM is excellent when it comes to taking pictures, it doesn’t give us any idea about the physical properties of the body.

2. This problem is solved by the Near Infrared Spectrograph (NIRSPEC). It operates over a wavelength range of 0.6 to 5 microns. A spectrograph is used to disperse light from an object into its spectrum. Different elements have their own characteristic spectra. Analyzing the spectrum of an object can tell us about its physical properties, including temperature, mass, and chemical composition. It reveals a plethora of information about the body being observed.

Spectroscopy: Emission and Absorption Spectra of various elements

The most significant drawback of using spectrographs is, the mirror must stare at them for hundreds of hours in order to collect enough light to form a spectrum. To the rescue comes JWST’s very own micro shutter system made of 250 thousand shutters. It controls how light enters the NIRSPEC. It has been developed by Goddard scientists. It allows us to observe hundreds of objects at a time saving a lot of time and resources.

3. FGS (Fine guidance sensor) – Different parts of the Universe can be brightly illuminated. To capture the relevant light, the telescope has to constantly be directed at different targets. This is achieved by a fine guidance sensor (FGS). It allows Webb to point precisely so that it can obtain high-quality images. FGS is a "guider," which helps point the telescope. Canadian scientists developed the near-infrared imager and slitless spectrograph (NIRISS). It is used to investigate exoplanets, detect first light and find out more about the physical characteristics of the observed body. FGS/NIRISS has a wavelength range of 0.8 to 5.0 microns. It is a specialized instrument with three main modes, each of which addresses a separate wavelength range.

FGS and NIRISS

Farther the source of light, the more red-shifted its wave and longer its wavelength. The Mid-Infrared Instrument (MIRI) is equipped with a camera and a spectrograph. It works with longer wavelength infrared light, in the mid-infrared region of the electromagnetic spectrum. Longer wavelengths can penetrate thicker dust clouds. MIRI covers the wavelength range of 5 to 28 microns. Its sensitive detectors allow it to see the red-shifted light of distant galaxies, newly forming stars, and faintly visible comets as well as objects in the Kuiper Belt. Due to its objective of working with longer wavelength infrared lights, it is important to be careful that it doesn’t start registering its own heat. The temperatures have to be kept below 6.7K. A special cryocooler that uses helium is used to keep it cool.

Near, Mid, and Far Infrared photography

C. Webb also has a Sun Shield with dimensions being 21 m long and 14 m across. It is made of five layers. Each is made from a special film called Kapton, a material that can absorb high temperatures. Additionally, there are layers of aluminium and the first two layers also have doped silicon. The sun shield protects it from the heat of the Earth, Moon and Sun.

Sun Shield

D. The Spacecraft Bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

Spacecraft Bus and other parts of JWST

E. Other elements include:

The momentum flap balances the solar pressure on the sun shield, like a trim flap in sailing. It's not adjustable in orbit, but it is while it's on the ground.

The Earth-pointing antenna sends science data back to Earth and receives commands from NASA's Deep Space Network.

The solar array is always facing the sun to convert sunlight to electricity to power the Observatory.

The star trackers are small telescopes that use star patterns to target the observatory.

I would love to talk more but this article is turning out to be longer than I expected. I guess we would need another one to answer the remaining questions. Until then, let's enjoy some beautiful images taken by the Webb.

The image that broke the internet! Perfectly visible Gravitational Lensing

Pillars of Creation

Images that put our understanding of the Universe into question

One of the first few pictures by JWST. Look at this beauty!

Comparing Hubble and JWST

This article fills me with glee, excitement, and hopeful anticipation about the future. What were your reactions? Comment below.

See you soon.

Bis Bald.

The Star Factory

Nebulae

Nebulae or Nebula arguably offer the most breathtaking sights of the universe. The countless pictures of these colourful nebulae fill their viewers with awe. A nebula by its Physics and definition is so simple and elegant, it’s hard to contemplate its simplicity when placed beside its other counterparts. The word Nebula comes from the Greek word for cloud and like its name is a giant cloud of dust and gas in space.

As we have earlier seen, as far as the baryonic matter goes, about 99.9% of it in our Universe is composed of Hydrogen and Helium – the two simplest atoms conceivable, with a contribution of 75% and 24% respectively. These atoms are scattered across the Universe with an average density of 5 hydrogen atoms per m³. This density seems too insignificant for us to even fathom that they can interact or create something. But, thanks to the Universe, these atoms are not uniformly distributed. Some spaces accumulate them. These might have wandered and collected due to their combined gravitational pull. These atomic clusters have densities similar to that of gases. Essentially forming vast quantities of free-floating interstellar and intergalactic gases.

Gravity is one of the most fascinating forces of nature. It’s the weakest of the four forces and yet the predominant one. Whenever it is brought into the mix, it gives rise to all the structures that make up the Universe.

The large clouds of hydrogen and helium gas start concentrating under their own gravity giving rise to large opaque clouds. With time, these clouds become denser and denser prohibiting even light to escape. Thus, the basic structure is formed. Now, the question arises how do we see these opaque clouds that do not let light pass through them? Well, let’s see.

All the observable bodies that emit light do so either by emitting light of their own, or by reflecting light from another luminous body. When these opaque gas clouds called the Dark nebulae get concentrated enough, they start fusing by a process called nuclear fusion. This results in the formation of the core of stars and essentially turns the dark nebulae into a star factory. These stars emit light making the nebulae visible.

Light emitted by the stars is reflected back by the surrounding gas, illuminating them in the process. This type of nebulae is called Reflection nebulae. These stars emit radiation ranging from infrared, and visible light to ultraviolet. This light excites the surrounding gas and ionizes the hydrogen and helium atoms. These atoms when de-ionize produce a photon with a wavelength corresponding to the difference between the energy states. These nebulae are now essentially emitting light giving rise to Emission nebulae. A simple question that may arise is, can a nebula be both – an emission and a reflection nebula? And the answer is absolutely YES! These nebulas are what we call Diffuse nebulae. They do not have any strictly defined shape.

Reflection Nebulae

Emission Nebulae

Diffuse Nebulae

The aforementioned nebulae form one type of Nebulae amongst many other varieties. Let’s look at others, shall we? The secret to finding them lies in a very basic question. What happens to these nebulas after billions of years have passed? Well... the stars begin to die. The further classification of nebulas depends upon the mass of the dying star.

Stars with intermediate masses ranging from 0.7 M_s – 8.0 M_s (1 M_s = 1.99*10³⁰ Kg) form our next two kinds. A living star maintains a delicate balance between the radiation pressure pushing the mass outwards and the gravitation pull moving the mass inwards. When a star begins to die, it appears to enlarge due to mass leaving the surface. A star in its final moments collapses under its own gravity sending ripples that scatter the mass around it. These intermediate-mass stars do not have enough mass to produce a supernova. The star dies in a whimper and the core becomes a white dwarf. This white dwarf then illuminates the surrounding gas and forms what we call the ejected nebulae or the protoplanetary nebulae. These nebulae can be easily identified by their two protruding lobes. Its unique shape involves Physics that goes beyond this article. We may come back to it later but for now, let’s keep moving forward.

Menzel 3 - The Ant Nebulae

NGC 2346 - The Twin Jet

Given enough time, these nebulae settle into shape. The dying star’s unique characteristics determine its shape. The outer gas forms an elliptical or spherical shape giving it the appearance of a planet. Historically, in the absence of good telescopes, many of these nebulae were identified as planets giving it its misdirecting name planetary nebulae. These planetary nebulas are also referred to as stellar nebulae.

NGC 7293 - The Helix nebulae

NGC 1501

If the mass of the dying star is more significant than 8.0 M_s, the stars die in a giant explosion called a supernova. The supernovae cause most of the mass to be distributed at once. The supernova remnants form a ring-like structure at the exterior. At the core of these supernovae nebulae is a neutron star or maybe even a black hole depending on the mass of the dead star.

SN 1006

SN 1604 - Kepler's Supernova

Now let’s take a step back and revisit the dark nebulae. They are formed by dense gases which block the light going out or passing through them. They are only visible when they are in the foreground of a bright nebula - dark highly concentrated clouds present at the centre of brighter, larger, less concentrated clouds. 

These dark nebulas are the most active region of a nebula. With time, it clears and reveals a cluster of young stars. This nebulae essentially eats itself inside out.

The Pillars of Creation

The Horsehead Nebulae

As we near the end of this article, let’s have a fun exercise for us. At night, look at the sky and try finding the Orion constellation. Try finding the three central horizontal stars. Now, go diagonally, and identify the Orion tail. Do you see three stars forming a line? Well, psst, the middle one is not a star. It’s a nebula! Awe-struck, aren’t you? I was too. Now, today let us all be a part of it and be united in our little Cosmo nerds community.

The Orion Nebulae (Hint: See the red dot in the picture)

See you soon.

P.S. These are some of the nebulae painted by yours indeed. Try identifying them and let’s discuss what’s happening in the comment section below.

Nebulae 1

Nebulae 2

P.P.S. This was one of my favourite blogs to write. Hope you had as much fun reading it as I had writing it. 

Auf Wiedersehen! 

The Giants before Giants

 Primordial Black Holes

Hello friends, it’s been a long while since I wrote a blog. My apologies for such a long break. But don’t worry, today’s topic of discussion is the most asked about. Let’s talk about primordial black holes (PBHs). But before I begin, I’d suggest you read a few of my previous articles and brush yourself up. Do revise particle physics, gravitational lensing, the big bang theory, dark matter, and Hawking radiation. I’d provide the links as the words come by. Without further adieu, let’s start.

Primordial black holes are said to be the black holes that formed immediately after the big bang. The time of their birth and their nature could give us a pretty good idea about the nature of the early universe. If these theoretical bodies exist, they would be the first celestial body formed. Although they have not been detected and exist in some theories only, they are an excellent candidate for dark matter.

The Big Bang and the creation of Primordial Black Holes

For the formation of PBHs, there are three requirements and these requirements are the hindrances too to their existence. The formation of black holes requires extremely high density which is not a problem at the beginning of the universe. All mass that is known and unknown, was in a space lesser than the diameter of an atom. Well, the mass part supports our theory but we need a density differential too. Without direction, there would be no point for the mass to face or the direction in which to get attracted to. This lack of a density differential combined with cosmic inflation would prohibit its growth. Now, the homogeneous mass distribution does not have a direction to move to and its speed of expansion is much more than the speed it can conjure to get attracted to a nearby mass. All of these effects, seem to make the existence of a primordial black hole almost impossible. BUT, there is one thing we have yet not considered. The effects that come into place only when the sizes are this small, a.k.a, the quantum effects.

The universe in question is of the size of an atom or smaller than it. Quantum fluctuations rule the universe, creating a kind of static fuzz. This fuzz when subjected to cosmic inflation would have turned the littlest of distances to distances in light-years. Some of these fluctuations were intense enough to resist the local expansion of the universe and form a black hole.

Current observation of the oldest light, or the first light that appeared when the universe became suitable for its travel, the cosmic microwave background radiation shows tiny differences in density of matter from one point in space to the next. Working backwards, these spaces would have been much more nuanced.

Cosmic Microwave Background Radiation

PBHs formed in a radiation dominated era. Most of its mass must have come from non-baryonic mass energy. A thing worth noting is when we talk about baryonic mass, normally, we do not mean the baryonic particles we studied in particle physics. A justified cause for particle physicists' angst, but we use baryonic mass to talk about particles that formed in the Big Bang nucleosynthesis. This mass encompasses the masses of light elements that formed, like hydrogen, helium, and small amounts of lithium and beryllium. By observing the current ratios of these elements we can infer the total density of the baryonic matter, and we get 4% or so of the total density of the universe corresponding to regular matter.

The regular black holes have evolved from this baryonic matter as the universe grew, but this is just the case of matter redistribution and has no effect on its overall density. So, we call these regular black holes baryonic black holes. Since the PBHs evolved before Big Bang nucleosynthesis and after inflation, they are called non-baryonic black holes. This would account for 27% of the unaccounted mass in the Universe, making it an excellent candidate for dark matter. But, in the more real sense we know these black holes were formed from electrons, protons and neutrons and protons and neutrons are baryons. Thus, these black holes are not non-baryonic, they are just called that. And, not to forget they might have existed even before, the baryons were formed at all.

The current candidacy for black holes is not new. It has been around for a while and the mystery is close to being solved. Primordial black holes belong to the class of massive compact halo objects(MACHOs). They are nearly collision-less, stable if sufficiently massive, have non-relativistic velocities, and are formed very early in the history of the Universe.

Let’s explore PBHs candidacy for dark matter a bit more.

Various cosmological events allow for a very small range of masses for the existence of dark matter. PBHs that originally had masses in the range of 10²¹ Kg would have vanished till now in what we call Hawking Radiation. Hawking Radiation is a phenomenon we have extensively discussed before. In short, it is a process by which black holes lose mass and sooner or later cease to exist.

Hawking Radiation

A heavy massed black hole mass when passes in front of a large luminous body we observe what we call gravitational lensing. Depending upon the masses it could even be twinkling or micro-lensing. This phenomenon rules out a few more mass ranges.

Gravitational Lensing

Very heavy PBHs would affect the structure of star clusters, it would disrupt orbits of binary star systems or might fall into a neutron star causing it to explode. The lack of these events in sufficient numbers also rules out a lot of matter ranges.

After a lot of calculations, we have two possibilities. Lots of primordial black holes with masses similar to large asteroids or a small number of really big PBHs. Again, one more problem that comes up with big PBHs is feeding. With masses approximating twenty to a hundred times that of solar masses, it would have left its mark on the cosmic microwave background radiation. What do you think?

All right guys, I would conclude this article here. There are lots and lots of things I would have loved to discuss with you but the word limit doesn’t allow me to do so. Mention your questions in the comments and let’s keep discussing.

Auf Wiedersehen!

Image sources: Google Images

Unravelling the Universe one particle at a time

Dark Matter - II 

Hello readers, this article is in continuation with my previous article on dark matter. So, if you have not read it before I’d recommend you to do check it out. Without further ado, let’s begin.

We left the article with a series of unanswered questions. So, let’s pick the first one from the list. What can and cannot be dark matter? What are the possible candidates of dark matter? 

Dark Matter - A scientific bewilderment

What dark matter cannot be?

The abundant amount of light elements created during the big bang nucleosynthesis (production of nuclei other than those of the lightest isotope of hydrogen after the big bang during the earlier phases of the universe) can rule out the possibility that dark matter particles are baryonic (Baryons contain an odd number of quarks, minimum 3. Protons and neutrons are the most common example of baryonic particles). The nucleosynthesis depends strongly on the baryon-photon ratio. This is also supported by the observations of cosmic microwave background radiation.

1. The main baryonic candidates are the massive astrophysical compact halo object (MACHO) class of candidates. These mainly include brown dwarf stars, Jupiter-like planets, and 100 solar mass (1 solar mass = 1.989 * 10³⁰ Kg) black holes. Searches such as MACHO collaboration and EROS-2 have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy.

   

   How MACHOS can focus light via gravitational lensing

   
   
2. Next in line with the particles that can be ruled out from the equation is relativistic neutrinos. Neutrinos are expected to have been produced profusely in the very initial stages of the universe. Similar to the microwave background radiation which are the fossil remnants of the hot radiation which characterised the dense phase of the early universe, we also expect fossil remnants of neutrinos which now form a background. With an estimated density of about 150/cm³, per species and this summed up over all the six species, we expect a fossil neutrino background with a number density of 1000/cm³. Even with this density, and mass as low as 0.1eV to 0.01 eV, it would account for less than a per cent of the missing dark matter.

   

   Cosmic neutrino background

   
   
3. There are a few other proposals that can be easily ruled out from basic astrophysical considerations. Highly relativistic protons trapped in halos of galaxies is one of them. Other rejected baryonic candidates are brown dwarfs, old white dwarfs, neutron stars, stellar-mass black holes, solid H₂, dense cold molecular clouds in galaxies, etc.

   

   Brown dwarf

   

   Ancient white Dwarf Star

   

   Neutron star

   

   A black hole weighing 70 solar masses

   
   

   

   Giant interstellar hydrogen iceberg

   
   

   

   Molecular clouds in the neighbouring WLM galaxy

   
   

What are some possible candidates for dark matter?

The range of ideas when it comes to what dark matter can be has no shortage. Serious candidates with masses ranging from 10^-5 eV to 10⁴ solar masses have been proposed. That’s a whooping range of masses of over 75 orders of magnitude!
As we have already discarded the notion of baryonic matter to be the dark matter, we shift our focus to non-baryonic matter. The non-baryonic candidates are basically elementary particles that are either not yet discovered or have non-standard properties. There are many propositions for the possible candidates. Many of them, like axions, neutralinos, gravitinos or composites these have been theorised. So, let’s look into them one by one.

1. A fraction of a second after the Big Bang, the universe was so hot that new particles and anti-particles were created and destroyed all the time. Calculations show that a stable particle of mass near 100 GeV and interacting with weak forces will leave just about the right amount of “leftovers” to account for the observed dark matter density. In particle physics, the Standard Model says that each particle has a heavier partner of different spin but similar interactions. The lightest of these particles is stable in many cases, which is an excellent dark matter candidate. Many theories which talk about higher dimensions, talk of different dimensions altogether in which these particles are curled up. The lightest of these particles, e.g., Kaluza-Klein particle, make for excellent dark matter candidates.

   

   Kaluza-Klein Theory

   
   
2. There are other possible dark matter candidates which do not fit into the above framework. One of these particles is the axions. An explanation for why axion is a good candidate goes beyond the scope of this article. Its explanation requires a pre-requisite knowledge of various types of symmetries which we would talk about in later articles.

   

   Newly found quasiparticles mimic hypothetical dark matter axions

   
   
3. Primordial black holes have also been suggested as a possible candidate for dark matter. Primordial black holes are hypothetical black holes that were formed after the big bang. It is found that black holes in the intermediate-mass range of one solar mass to a thousand solar mass and sub-lunar black holes in the range of 10¹⁷ – 10²¹ Kg can still produce all the dark matter. There are many constraints to that and the mathematics to it is still blurry, but we can be optimistic.

   

   Primordial black holes just after big bang

   
   
4. We have a few exotic candidates that have been suggested – WIMPzillas, gravitinos, gluinos, Q-balls, Q-nuggets, SIMPS, etc. There is a range of possible dark matter models. One other model is that baryons can be ‘packaged’ in non-luminous forms. There is also evidence that much of the dark matter may be made up of as yet undiscovered particles with several experiments all over the world trying to detect these. Many of these particles are in the preferred range of 100 GeV to a TeV. There could be dark matter objects or clumps made up of these particles bound by their mutual self-gravity and limits have already been placed on the abundance of these objects.

   

   Gravitino - Warm dark matter

   
   
5. There are several new classes of dark matter objects. One of the favoured dark matter candidates called the WIMPs (weakly interacting massive particles) has masses from about 10 GeV to 1 TeV. It can gravitate to form a new class of objects in dark matter halos or around the galactic centre. The role of dark matter in planetary formation and evolution has been considered by several authors.

   

   Theories of dark matter

   
   
   
6. Another alternate candidate to standard dark matter is the mirror matter-type dark matter. They have the right properties to be identified with the non-baryonic dark matter in the universe and make for an excellent candidate. I hate to say it but we are not ready for an in-depth discussion on mirror particles just yet.

Keeping in mind that I do not have to make my articles very lengthy, I would finish this topic for today. Needless, to say this will open new horizons to our discussions and give us a lot more fundamental words to use for the future. I hope the article kept you engaged, made you curious and fascinated you. Because these are the seeds for an inquiring mind that is ready for facing the challenges of Physics head-on. And even if today was a boring lecture, well, there would always be parts that you just do not like so much. Don't be disheartened and consider it one necessary evil. With that note, I would take your leave. Hope to see you soon.

 Auf Wiedersehen!

Footnote: I have written mass in terms of energy, more specifically electron volts(eV) in the whole article. This way of representing masses with electron-volts has its roots in the famous mass-energy equivalence equation, E = mc². Dividing 1 eV energy with c² gives the mass in kg.

1eV = 1.6 x 10-19 joules (J)

The various prefixes like K, M, G, and T. stands for kilo, mega, giga, and terra respectively, with the orders of magnitude being 10³, 10⁶, 10⁹, and 10¹², respectively.